A centrifugal clutch typically includes a driver or impeller configured so as to be rotated by an external power or driving source, a hollow drum coaxial with and disposed about the driver and configured to drive an external load, and one or more shoes located within the drum that are rotated relative to the drum by the driver. The shoes are generally adapted to move radially into and out of frictional engagement with the inside of the drum. The frictional engagement with the drum provides load transmission between the driver and the drum.
The shoes are typically urged towards the center, away from the drum, by one or more springs. As the driver and the shoes rotate, the centrifugal force created by the rotation urges the shoes radially outward. When the speed of rotation is sufficiently high, the centrifugal force acting on the shoes overcomes the force of the springs, urging the shoes to move outward and engage the drum. The engagement of the shoes with the drum causes the drum, and thus, the external load, to rotate in combination with the shoes. The speed at which the clutch engages is, therefore, determined by a balance between the mass distribution of the shoes and the strength of the springs.
Centrifugal clutches are commonly used in the drive trains of machines powered by small internal combustion engines for producing varying amounts of horsepower. These types of clutches have particular use in lower horsepower machines, such as wood chippers and go-karts, which typically operate at up to about 20 hp. The clutch is typically set to disengage when the engine is idling, and to engage when the engine is generating sufficient torque to drive the load effectively.
However, since the engagement between the shoes and the drum is based on friction, there is always a certain degree of slippage that occurs. For example, when the centrifugal force first overcomes the spring force, the initial contact between the moving shoe and the stationary drum will result in slippage. As the speed of the motor increases, the centrifugal force should eventually produce sufficient friction to prevent slippage. However, until that speed is attained, there will be relative movement between the shoes and the drum. This slippage is necessary to some degree to provide for a gradual acceleration of the driven component.
In situations where the engine is operating at a fairly low speed the slippage may continue for some time. This produces a considerable amount of friction which, in turn, results in the generation of heat. The highly-resilient steel from which springs are typically made is not very heat-resistant. To compound this, the springs are necessarily under considerable stress since they are fighting the centrifugal force imposed on the shoes. It has been found in practice that a slipping centrifugal clutch can easily generate sufficiently high temperatures to cause the metal of the springs to creep. As the springs creep, their spring force decreases, which then allows the shoes to engage at lower rotational speeds. The heat can also become sufficiently high so as to change the temper of the metal, further weakening the spring.
A decrease in the speed at which the clutch engages is generally undesirable, and in some cases unacceptable. For example, if a spring relaxes considerably, the shoes can engage the drums at even an idling speed. This can cause the driven component to begin to move when the engine is idling. In some instances, this can be an unacceptable safety concern. For example, in a go-kart, movement of the vehicle at idle, when people are typically getting into or out of the go-kart, can be very dangerous. Alternatively, the engine may stall if the clutch engages before the engine is producing sufficient torque to drive the load.
In one conventional centrifugal clutch, a single garter spring is used to hold the shoes radially inward. The garter spring is seated in slots, extending in a generally circumferential direction, on one side of the shoes. The garter spring is located close to the frictional surfaces of the shoes. As a result, the heat generated by the shoes transfers readily to the spring, reducing its operating life. It has been found with one centrifugal clutch of this type used in a transmission of a go-kart that the garter spring can relax sufficiently to affect the performance of the vehicle in as little as 30 minutes of driving.
In another conventional centrifugal clutch, separate coil springs are attached between each shoe and its neighbors. In this clutch, the ends of each spring are hooked into holes formed in the shoes. These hooks, and especially the bend where the hook joins the coiled part of the spring, are the most highly stressed parts of the springs. Also, because of their location relative to the shoes, the hooks heat up more than the rest of the spring. As a result, the heat causes the material to relax, allowing the hooks to deform and reducing the spring force.
Conventional centrifugal clutches have limited life, requiring frequent maintenance in order to maintain proper power transmission. A need, therefore, exists for an improved centrifugal clutch design which extends the life of the springs used in the clutch.